Packaging films which have a matte-finish are aesthetically useful for many applications. A matte-finish can be used to provide a desirable satin paper-like finish when employed as an outer surface of a packaged product. Films having a matte-finish surface are particularly useful as outer webs of laminated composite packaging films such as those used to package snack foods, bakery products, and other confectioneries such as candy.
Matte-finish films are also useful as tape base films where a pressure sensitive adhesive is applied to the reverse side of the film and an overlacquer is applied to the matte-finish surface. The use of such films eliminates the need for an expensive matte coating step prior to application of the overlacquer. Matte-finish films can also be used in graphic arts applications where a non-glossy satiny-paper look is desired on an exposed surface, such as in book covers. In addition, matte-finish surfaces can generally be written on with a variety of printing inks.
Many matte-finish films are currently available but are manufactured out-of-line as coatings which are subsequently applied to an existing film surface. The matte-finish coatings generally comprise a combination of ingredients which yield a pressure sensitive adhesive layer. Such films are taught, for example, in U.S. Pat. No. 5,342,339. Such a process involves double film handling and is uneconomical compared to coextrusion techniques.
An alternative route to a matte-finish film surface has been achieved by exposing one or more surface of a substrate to an embossing or impression roller which physically changes the surface roughness of the substrate. Many embossing techniques are known.
Within the field of biaxially oriented polypropylene (BOPP) films, it is known that ethylene-propylene block copolymers can be used to achieve a matte-finish appearance. Such technology is especially popular in Japan. Domestic matte-finish BOPP products are also available but the exact compositions and methods of obtaining a matte-finish on these films are not known.
U.S. Pat. No. 4,996,096 teaches a coextruded non-embossed matte-finish film made from a polymer blend which consists of a tackifier, polyisobutylene (10-40% by weight), high density polyethylene (50-90% by weight), and optionally polypropylene homopolymer (0-40% by weight). This matte layer is preferably coextruded on a pigmented, non-transparent polyolefin core layer.
Many packaging films are used in combination with cold seal adhesives and cohesives to form air-tight hermetic packages. For many packaging applications, films employing cold seals and having a matte-finish are desirable.
Cold seals for packaging films may comprise blends of natural rubber (cis-polyisoprene) latex with a synthetic polymer adhesive such as an ethylene vinyl acetate (EVA) or acrylic. Such cold seals are water based emulsions (latexes) which may also contain minor ingredients such as wetting agents, stabilizers, defoamers, antiblock agents, colorants, etc. Cold seals are usually applied by either direct gravure, roller coat, spray or Meyer rod to a flexible packaging substrate in a pattern about one half inch wide surrounding the anticipated perimeter of a product to be wrapped. The volatile components of cold seals, such as the main volatile--water, are normally removed by forced hot air. Thereafter, a solid layer of material, between about 2 to 4.5 pounds per ream, is left remaining on the substrate surface.
One method of cold sealing involves joining the cold seal to itself and forming a packaging closure by applying pressure for a period of time at ambient temperatures. This type of closure method is commonly referred to as "cohesive" or "self" sealing. This, and other types of cold seals are particularly useful in wrapping products that are heat sensitive, for example, confectioneries, ice cream, bakery products, and dairy products such as cheese.
Cold seal cohesives are becoming the sealing medium of choice over traditional heat seal materials when faster packaging speeds and improved seal quality and consistency are desired. Cold seals also have the advantages of not causing the packaging substrate to melt during equipment shut downs or stoppages. In addition, cold sealing can save energy and time when compared to the operation, maintenance and control of heat sealing equipment.
Configurations for flexible cold seal packaging laminates comprise an outer web constructed of 50 to 100 gauge biaxially oriented polypropylene (BOPP) film having a slip and antiblock modified cold seal release layer formed thereon. Reverse printed ink may then be applied to the BOPP film surface opposite the release layer, and a laminating adhesive or polyethylene extrusion is typically applied between the outer web and an inner web. The inner web typically comprises a cold seal receptive BOPP or polyethylene terephthalate (PET) metallized, clear or opaque white film having a thickness of from 32 to 150 gauge. A cold seal latex cohesive is applied in a registered pattern onto the exposed surface of the inner web.
Flexible single web cold seal packaging configurations for confectioneries are also known. Single web configurations comprise a 50 to 150 gauge cold seal receptive BOPP or PET metallized, clear or white opaque film having surface printed ink applied to one side thereof. A cold seal latex cohesive is applied in a registered pattern to an opposite side thereof. An outer surface is formed over the surface printed ink and comprises an overprint, varnish and/or overlacquer.
For the aforementioned configurations and others, cold seal cohesives are generally applied to the inside (cold seal receptive surface) of a film lamination or to a single web film. In each case, the outer film surface (facing away from the cold seal) must repel adhesion to the cold seal on the inner web so that the roll stock may be unwound when it is used to wrap a product. In the case of a single web film, the surface opposing the cold seal cohesive is typically coated with a polyamide type overlacquer to provide sufficient release from the cold seal, i.e., to prevent roll blocking.
In order for a cold seal to be useful as a closure, the cold seal emulsion must have good wetting properties when applied to the cold seal receptive surface of the packaging film. In addition, the cold seal must exhibit strong anchorage, i.e., adhesion to the cold seal receptive surface, when dried. Absent sufficient anchorage, the cohesive can be stripped from the receptive layer resulting in breakage of an otherwise intact hermetic seal.
A need therefore exists for an improved cold seal receptive layer which provides excellent cold seal cohesive anchorage for a variety of cohesive compositions.
A need also exists for a matte-finish packaging film which exhibits an improved adhesion between a cold seal cohesive and the cold seal receptive surface.